Abstract

The occurrence of intersexuality was investigated in Gammarus fossarum populations from five different streams in Germany in the years 2000-2003. In the streams Lockwitzbach and Körsch sampling sites were upstream and downstream of sewage treatment plant effluents, in the three other streams an effluent was absent. In the stream Lockwitzbach the median frequency of intersexes at the two sampling sites was 7.1% and 13% over two years. In the streams Körsch and Zschonerbach the frequency of intersexes was lower with 0.2 % and 0.8% at the two sampling sites at the Körsch, respectively, and between 0.3 % and 0.7% at the Zschonerbach. Because in the Goldersbach and the Friedrichsbach in the absence of effluents the frequency of intersexes was 3.2% and 24%, an impact of chemicals from municipal sewage treatment plants can be excluded. As the phenomenon occurred in three of five streams but can easily be overlooked, it may be much more common than previously assumed. At the upstream sampling site of the Lockwitzbach with a high frequency of intersex the affected specimens were larger compared to females. An exposure of gammarids from streams with low intersex frequency to water from streams with high intersex frequency led to a significant, time dependent increase of intersex frequency. Thus, although there are reports about induction of intersex by photoperiod, a factor which plays no role in the experiments performed here, it is concluded that some factor in the water - may be a chemical - is responsible for the induction of intersex in G. fossarum. At the moment it is unclear whether an anthropogenic or natural factor is responsible and which role environmental conditions play.